As with any device incorporating an electric motor, the disk drive motor generates heat as it operates and thus warms the motor and related parts of the device, thereby creating thermal expansion of the motor components and the disk drive components. This thermal expansion, focusing on the spindle or rotor of the disk drive motor and a hub which is fitted over the spindle of the motor for carrying the rotatable disks for data storage, is both radial and longitudinal expansion or growth.
In order to insure the most reliable connection between the hub and the spindle of the disk drive and eliminate other attachment hardware, the hub is typically shrink fitted onto the drive motor spindle. The shrink fit and assembly of the hub spindle insure an interference fit and reliably couples the hub to the spindle for rotation with the drive motor spindle. Alternatively, the interference fit may be accomplished by a force fit.
With an interference fit between the hub and the spindle of the drive motor, the connection is totally dependent upon friction between the two surfaces with the forces normal to the surface at any particular point provided by the combination of stretch of the hub or compression of the spindle.
Inasmuch as the assembly is dependent upon the surface-to-surface engagement and friction, thermal expansion and thermal growth mismatch will occur both radially and longitudinally. The radial change in forces will be generated by the relatively larger expansion of the hub in a radial direction than the radial expansion of the spindle. The longitudinal forces will be created as a result of the mismatch in the expansion rates between the spindle and the hub, typically made from a ferrous material such as stainless steel and aluminum or aluminum alloy, respectively. As the parts are warmed by operation of the disk drive motor, there will be a force created along the interface between the spindle and the hub in a direction parallel to the spindle axis of rotation. The frictional forces between the hub and the spindle will act to resist any movement between the hub and the spindle.
Since the hub and the spindle expand at different rates due to the dissimilarities of the materials, some portions of the hub will attempt to move relative to the spindle along the direction of the forces generated at the interface which lies substantially parallel to the axis of the drive motor and the spindle. The amount of force which may be generated between the hub and the spindle will vary from region to region due to the frictional engagement between the hub and the spindle, but the force will be inherently limited by the coefficient of friction between the two surfaces and the forces normal to those surfaces, for any unit of area. As the temperature of the hub and the spindle rise during operation of the disk drive, longitudinal forces will increase; and at some point, the forces generated by the longitudinal thermal growth mismatch of the spindle and hub will exceed the resistance provided by friction between the two components. Whenever that occurs, there will be some translation between the two parts at that particular point. Typically, on any longitudinal line or zone of contact lying on the interface surfaces between the hub and the spindle parallel to the drive motor axis, there will be one point where no movement will occur as well as points on one or both sides of that point where such relative movement does occur. The point where the movement does not occur may be referred to as an anchor point due to the anchoring which results, at that point, from the frictional forces.
When the anchor point for one portion of the hub/spindle interface is not located on a line or within a very narrow band which is defined by the rotation of a radius about the axis of rotation of the disk drive motor, there will be movement of various portions of the hub in different directions in varying amounts, all dependent upon and relative to the location of the anchor points around the spindle/hub interface. With different directions and different amounts of movement, the result will be a cocking or tilting both of the hub and also the disk stack attached thereto relative to the spindle and rotation axis. This tilting will cause the hub to assume an off axis position with respect to the axis of rotation of the disk drive motor and will thus cause a wobble of both the hub and the disks as they rotate and track misregistration.
Prior efforts to accommodate thermal expansion and thermal mismatch have been attempted by leaving a gap between the inner cylindrical surface of the disks or the hub and the spindle of the disk drive motor. This permits radial expansion of the spindle without distorting the hub; and where the gap is left between the hub and the spindle, it also tends to overcome the problems presented by a interference fit between the hub and the spindle. However, this design arrangement requires clamping of the hub to the disk and, accordingly, results in additional hardware, mass, and balance problems. Further, leaving a gap between the hub and the spindle to accommodate thermal expansion at varying rates compounds problems of insuring concentricity of the disks with the spin axis of the spindle and hub. However, leaving a gap between the spindle and the hub permits radial expansion of the hub relative to the spindle without distortion of the hub. A similar problem typically is not encountered with respect to the hub and the disks inasmuch as the disks and the hub are matched with regard to the coefficient of expansion of the material, thereby eliminating distortion caused by mismatched thermal expansion.
Prior attempts to maintain the concentricity and accommodate thermal mismatch have utilized longitudinal ribs parallel to the axis of the disk drive motor to support the disk and spacers around the spindle. Sleeves and ribbed sleeves oriented longitudinally with respect to the spindle have been used both to surround the spindle and to space the hub from the spindle. In some cases the passages between the spindle and the hub or the spindle, disks and spacer rings have been used for cooling air passages in an attempt to reduce the temperature rise of the disk stack and its spacer rings and thereby reduce the amount of thermal expansion that must be accommodated.
None of the above approaches address the longitudinal thermal expansion which is encountered in the hub and which creates a thermal growth mismatch and possible movement of part of the hub with respect to the spindle. Accordingly, none of these approaches address the problem of cocking or tilting of the disk stack as a result of thermal expansion of the disk drive components during operation. Any tilting or cocking of the disk stack relative to the axis of the drive motor can result in track misregistration and/or wobble, both of which will cause the disk drive to malfunction with respect to the reliable writing and reading of data on the magnetic disks.